Snap-acting thermostatic element and method for making same

ABSTRACT

A THERMALLY RESPONSIVE MULTIMETALLIC ELEMENT FORMED WITH A DISHED ARE TO MAKE IT SNAP-ACTING IS PROVIDED WITH A PLURALITY OF DEFORMATIONS FORMED IN PAIRS IN THE DISHED POSITION OF THE STRIP WITH EACH PAIR COMPRISING TWO POINTS AND RIDGE CONNECTING EACH POINT IN A PAIR WITH A CORRESPONDING POINT IN THE ADJACENT PAIR. THESE DEFORMATIONS ENHANCE THE SNAP CHARACTERISTICS OF THE ELEMENT AND REDUCE CREEP PRIOR TO SNAP ACTION OF THE ELEMENT FROM ONE CONFIGURATION TO ANOTHER AT A CRITICAL TEMPERATURE.

Feb. 9, 1971 H. D. van 3,562,690

SNAP-ACTING THERMOSTATIC ELEMENT AND METHOD FOR MAKING SAME Filed April 28, 1969 Inven tor, Hamlet D. Vezza 6 f Z- Attzy.

United States Patent l 3,562,690 SNAP-ACTING THERMOSTA'IIC ELEMENT AND METHOD FDR MAKING SAME Hamlet D. Vezza, Seekonk, Mass., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Apr. 28, 1969, Ser. No. 819,868 Int. Cl. Htllh 37/54, 71/16 US. Cl. 337-89 9 Claims ABSTRACT OF THE DISCLOSURE A thermally responsive multimetallic element formed with a dished area to make it snap-acting is provided with a plurality of deformations formed in pairs in the dished portion of the strip with each pair comprising two points and ridge connecting each point in a pair with a corresponding point in the adjacent pair. These deformations enhance the snap characteristics of the element and reduce creep prior to snap action of the element from one configuration to another at a critical temperature.

This invention relates to improvements in snap-acting thermostatic elements and the like.

It is an object of the invention to provide snap-acting thermostatic elements which have improved characteristics, including elements which have an improved range of calibration and which maintain their calibration for a longer period of time. Another object is the provision of a snap-acting thermostatic element which exhibits less creep prior to snap action, that is, there is less moving of the element prior to the reaching of a critical temperature, at which temperature the element rapidly and suddenly snaps from a first configuration to a second configuration and vice versa. Still another object is the provision of a snap-acting thermostatic element which is constructed in such a way that with modern mass construction techniques a greater percentage of the elements made can be utilized. Yet another object is the provision of a snap-acting thermostatic element which expends increased force when snapping from one configuration to another. Other objects, features and advantages will be apparent from the following description, taken in connection with the appended claims and attached drawings:

FIG. 1 is a perspective view of a thermostatic element before forming in accordance with the invention;

FIG. 2 shows the same element after it has been formed in accordance with the invention and showing means to accomplish this formation; and

FIG. 3 is a side elevation partly in section of a switch made in accordance with the invention.

In the accompanying drawings, in which one of the various possible embodiments of the invention is illustrated, similar reference characters indicate corresponding parts throughout the several views of the drawings. Dimensions of certain of the parts as shown in the drawings have been modified and/or exaggerated for the purpose of clarity of illustration.

In FIG. 1, reference numeral designates a thermostatic element which is in the form of an elongated strip having at one end a generally rectangular mounting portion 12 with an aperture 13 provided therein and at the opposite end, a portion 14 which mounts an electrical contact 17 by conventional means such as welding. A typical size for the element shown is in the order of long by wide. This thermostatic element 10 may, for example, be formed of bimetallic material having a layer of metal of a comparatively high thermal coeflicient 3,562,690 Patented Feb. 9, 1971 of expansion on its lower side as viewed in FIG. 3, and a layer of metal with a lower coefficient of expansion on its upper side. Because of the thinness of the element 10, the two layers are not indicated on the drawings. To provide snap action, element 10 is formed with a curved portion, that is, it is dished upwardly as indicated in FIG. 3 to cause the element to snap between the solid and dotted line configurations shown in FIG. 3. Reference numeral 10' indicates a strip formed as a snap-acting element. When the element is cool, it is in the position shown by the solid line. Upon heating it to a predetermined critical temperature, the greater expansion of the lower layer of metal relative to the upper layer of metal causes the element to snap upward to the FIG- 3 dotted line position. Thereafter, on cooling of the strip to another predetermined critical temperature, the strip will snap back to the solid line position.

Element 10 can be cantilever mounted on a support such as electrical insulating base 15, by conventional securing means, such as screw 16, which is inserted through aperture 13 into base member 15. Movable contact 17 mounted on portion 14 of element 10 is adapted to engage and disengage with stationary contact 18 mounted on base 15 as indicated by the .solid and dotted line positions in FIG. 3, and hence, with terminals (not shown) connected to stationary contact '18 and portion 12 of element 10, controls the opening and closing of an electrical circuit. It should be noted that other types of thermally responsive sheet materials may be employed such as other multimetallic structures as well as elements of other shapes.

One of the problems associated with prior art thermally responsive members of the type described which are snap-acting is that prior to snap action as the critical temperature is approached, the members begin to gradually flex or creep. That is, the distal portion 14 of strip 10 tends to move upwardly as seen in FIG. 3 as the temperature increases prior to reaching the critical snapping temperature. It is obvious that this is an undesirable feature since it makes accurate calibration more difficult. Further, the opening of a circuit with such an element is slower and more laggard, thereby encouraging the generation of arcing which is deleterious to the life and calibration of the device. In other words, even though the prior art element snaps, oftentimes the circuit will be opened by the element creeping open prior to the critical temperature at which the element snaps. There is no predictable or precise moment when the circuit will be opened.

Another problem that is involved with prior art elements is that the amount of creepage varies widely among various elements. This results in a requirement that the elements be carefully selected for any application and tested to see that the amount of deflection is not excessive for the application. Necessarily, many strips are rejected because the creeping deflection is excessive.

Various attempts have been made to overcome these shortcomings in the prior art, as in US. Pat. No. 3,431,- 527, which issued Mar. 4, 1969, assigned to the assignee of the instant invention. In this patent, a tongue is struck 7 from the thermostatic strip and pressure is applied to the tongue to force engagement of a movable contact mounted on the strip with a fixed contact mounted on a base. This tends to eliminate creeping prior to snap action of the strip; however, it requires additional parts and makes the switch in which it is contained bulkier and necessitates more labor in assembly to effect the adjustment of the desired amount of force on the tongue.

In accordance with the instant invention, creep action is greatly reduced without using additional component parts. A preferred way of making elements in accordance with the invention will be described with particular reference to FIG. 2. Forming die 20 and die punch 22 are used to form the dished portion of the strip. Included in the forming die 20 are a plurality of interconnected recesses such as bores 24, 26, 28. The intersections of these recesses are shown at 25a, 25b, 27a and 2712. Forming die 20 and forming punch 22 are movable relative to one another as by plunger 23 attached to punch 22. The bottom surface of punch 22 which is concave and the top surface of die 20 which is convex have the same contour except for the recesses formed in die 20. Basically, the surface is made up of two radii, one causing curvature along the length of the die and punch (arcs 31 and 30 of die 20 and punch 22 respectively), and the second causing curvature along the width of the die and punch (arcs 33, 32 of die 20 and punch 22 respectively).

In order to make strip into a snap acting element, it is placed between punch 22 and die 20. Punch 22 is brought downwardly as indicated by the arrow in FIG. 2, forcing the strip to conform to the matching surfaces of the die and punch. However, because of the recesses formed in die 20, points corresponding to the intersection of the recesses at 25a, 25b and 27a, 2712 are deformed in the dish formed portion of the strip at 25a, 25b, 27a, 27b respectively. Point 25a is connected to 27a by an arc shaped ridge 34 while point 25b is connected to 27b by a similar arc shaped ridge 35. Shorter arcs, 36-39 inclusive, extend from the points in directions opposite to the arcs 34 and 35. If extended, arc 36 would meet arc 37 and are 38 would meet 39. Since strip 10 is flat before forming the force exerted by punch 22 is more effective along the latitudinal axis 4, therefore the ridges connect points 25b, 27b and 25a, 27a, but not 25a, 25b, and 27a, 271;. Thus the ridges join corresponding points in adjacent pairs.

The points are located in pairs, each point in a pair preferably equidistant from longitudinal axis 2 and" each pair equidistant from a chosen latitudinal axis 4. The location of axis 4 controls the effective center of snapping of the strip along longitudinal axis 2 as can be seen in FIG. 3. It will be understood that more than two pairs of points can be provided if desired. While in some cases, one pair can be employed to achieve the objects of the invention, it is necessary to use a plurality of deformations. While in the description the deformations are formed in the element in the same operation as the forming of the dished area, it will be appreciated that a separate forming action to impart the deformations could be employed if desired. Further, it is within the purview of the invention to provide a greater number of recesses or to employ shapes other than circular, although it is preferable that they be smooth curves.

Snap acting elements made in accordance with the invention have many advantages, inter alia, they are more dependable, more predictable, have longer life, are easier to calibrate and have a greatly improved yield rate, improved by as much as 95 percent in some cases.

It will be understood that snapacting elements are made to snap at a selected temperature, e.g., 250 F.; however, there is necessarily some variation in the temperature at which the element snaps from one time to another. This variation of range is typically in the order of to F. for elements not having deformations; however, it has been found that elements made in accordance with the invention having a temperature range nearly half that, approximately a 13 F. range. Further, the particular range is maintained for a longer period of time; that is, for more cycles, giving it a longer useful life than prior art elements. Yet another advantage is that elements having the deformations are easier to calibrate. One way to calibrate an element after it has been formed is to apply pressure to the element by a calibrating pin through the' movable contact. The pin is moved a certain distance to effect the desired calibration. Without the deformations, this distance is very short and critical making it difficult to obtain accurate calibration. However, to obtain the same temperature calibration point for an element made in accordance with the invention, an operator moves the pin approximately twice the distance. This makes it possible for a less skilled operator to calibrate an element even more accurately and in less time than a prior art element since there is less criticality involved in the movement of the pin per unit of distance.

Still another advantage is achieved since the deformations tend to prevent snap action for a longer period of time until the forces built up in the element concomitant with the temperature rise finally cause the element to snap, resulting in a greater amount of force exerted by the element. This could be utilized, for instance, to transfer motion by a motion transfer member from the snapacting element to some other element to effect actuation or deactuation of a circuit isolated therefrom.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

It is also to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phrascology or terminology employed herein is for the purpose of description and not of limitation.

As many changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

I claim:

1. A snap acting thermostatic element comprising:

a thermostatic strip formed with a dished surface portion responsible for the snapping of the element and plurality of deformations formed in pairs in the dished portion of the strip, each pair comprising two points and a ridge connecting each point in a pair with a corresponding point in an adjacent pair whereby the snap acting characteristics of the element are improved and creep movement prior to snap action is reduced.

2. An element as defined in claim 1 in which the strip is an elongated bimetallic element.

3. An element as defined in claim 1 having a longitudinal axis in which each deformation of a pair is located on opposite sides of the longitudinal axis and equidistant therefrom.

4. An element as defined in claim 1 in which the degree of deformation of the points is greater than of the ridges.

5. An element as defined in claim 1 in which the ridges are are shaped.

6. A method of making a snap acting thermostatic element comprising the steps of (a) providing a multimetallic sheet of material,

(b) forming a dished surface in the sheet, and

(c) forming a plurality of deformations simultaneously in the strip.

7. A claim according to claim 6 in which steps (b) and (c) are formed simultaneously.

8. A claim according to claim 6 in which the deformations are formed in pairs along an axis of the sheet.

9. An electrical switch comprising a base of electrically insulating material,

a thermally responsive bimetallic element having two ends formed with a dished portion causing it to snap act at a critical temperature from one configuration to an opposite configuration,

a movable contact mounted on the element,

6 a stationary contact mounted on the base, the element References Cited cantilever mounted on the base such that the mov- UNITED STATES PATENTS able contact moves into and out of engagement with the stationary contact when the element snap gii 21 acts, and 5 2,487,684 11/1949 Smith 337365 pairs of deformations provided in the element, each deformation of a pair located on opposite sides of BERNARD A, GILHEANY, P i E i an axis on the element and equidistant therefrom, D M MORGAN Assistant Examiner each pair comprising two points and a ridge connecting each point in a pair with a corresponding 10 U5, CL X R point in an adjacent pair. 337111 

